The present invention relates generally to semiconductor processing and, more particularly, to processes of forming buried high-resistivity layers and high-quality surface single crystal silicon layers with high electron mobility.
Electron mobility in materials, substrate resistivity, and device size are three factors which affect speed (the speed of integrated circuits). Raising device speed can be achieved by increasing electron mobility in materials, increasing substrate resistivity, and shrinking the device size. For a long period of time, people have been looking forward to finding a new material with high electron mobility, high resistivity substrate, and excellent crystal microstructure in order to manufacture ultra high-speed, very large scale integrated circuits (VLSI).
Czochralski (CZ) silicon is relatively inexpensive and is the best available material with acceptable microstructure. CZ silicon is used worldwide for VLSI manufacturing at present. But mobility in CZ silicon cannot reach its theoretical limit value (i.e., intrinsic mobility) because of the existence of oxygen (about 1.times.10.sup.18 cm.sup.-3) and oxygen related defects. Thus, all attempts previously have failed to raise the resistivity of CZ silicon to higher than 100 .OMEGA.cm.
Silicon-on-insulator (SOI) technology is reaching the point of actual application to a manufacturable high-speed integrated circuit (IC) in recent years. One of the more successful methods of SOI production is the formation of a buried insulating layer by implantation of oxygen or nitrogen. But, substrates formed by oxygen implantation have the following disadvantages: The damage in the surface region due to oxygen ion bombardment is comparatively serious because the volume of oxygen ion is comparatively large. In other words, oxygen implantation with a dose of above 1.times.10.sup.18 cm.sup.-2, that is too high, leads to many defects at the surface region and the surface layer contains some defect-related oxygen precipitates produced during subsequent annealing. And these defects and oxygen precipitates seriously affect the shrinkage of the device size. In addition, the surface mobility in silicon wafers decreases substantially after oxygen implantation and subsequent annealing.
Furthermore, other approaches in the prior art which have attempted to improve SOI substrate by implantation such as, for example nitrogen implantation cause problems Similar to oxygen implantation. And even Silicon-on-Sapphire (SOS) substrates in which Si films are grown heteroepitaxially on insulating sapphire substrates (i.e., SOI without implantation), surface silicon layers contain the many defects because of lattice mismatch between Si and Al.sub.2 O.sub.3.
Another kind of important material used to manufacture semiconductor devices is GaAs. The two most favorable properties of GaAs are high electron mobility and semi-insulating substrate which result in high speed of GaAs ICs. Unfortunately, up to now control of crystal microstructure of GaAs material is still difficult and costly.
Many materials for use in manufacturing high-speed ICs have been investigated in the past years. They can be grouped into three main categories: GaAs, CZ silicon, and SOI substrate (including SOS substrate), all of which have serious disadvantages. For GaAs, present technology for controlling crystal microstructure manufacture is expensive. For CZ silicon, electron mobility and resistivity of the substrate are low. For SOI substrate, numerous defects at the surface region results in low electron mobility and poor crystal microstructure.